The Equipment

Bento, while less expensive than buying daily lunches, has a few start-up costs: equipment.

The basic accessory is the box itself. This can be anything from my adorable Sanrio and San-X boxes to the simpler box that I gave TheBoy for Christmas to the manly Mr. Bento by Zojirushi, a Japanese company that designs high-end (awe-inspiring, magic) rice makers and other expensive toys that I want.
I bought my two boxes on eBay for a little less than $20US, which paid for the two separate boxes and shipping for both from Japan.

The other tool usually considered a necessity with bento preparation is a rice cooker. A countertop rice steamer does not have to be the $200US monster sold by some companies -- it can cost as little as the $20US variety sitting on my kitchen counter.

Past those two pieces, you hardly need any initial investment at all.